The present invention is directed to efficient multistep processes for the preparation of compounds of the formula (6) and (6'), as shown in Scheme 3 below; and to certain of the intermediates, specified by the general formulas (8) and (9) below, which find special value in these multistep processes. The compounds of the formulas (6) and (6') are useful as precursors of the various penem antibiotics specified by the formula (7) and (7'), also shown in Scheme 3 below.
Heretofore, a number of processes have been reported for the preparation of penem antibiotics substituted at the 2-position with an alkyl group or a thioether group, --SR.sup.2, as found in the formulas (6), (6'), (7) and (7') below. For thioether compounds (6) and (7), two of the more general of these processes are illustrated in Schemes 1 and 2. In Scheme 1, an alternative intermediate to the silver salt of the mercaptan is the mercaptan itself, reportedly obtained by Zn/H.sup.+ reduction of the tritylated thiol (Girijavallabhan et al., J. Antibiotics 39, 1182 (1986); U.S. Pat. No. 4,584,133). Menard et al., U.S. Pat. No. 4,272,437, has also described processes related to those of Scheme 2, which were applied more generally to the synthesis also of the compounds (6') and (7'). For example intermediates of the type (K) are reacted with an acylating agent such as EQU R.sup.5 R.sup.6 CH--COCl
to form compounds structurally related to (L), which in turn are heated to close the ring and thus form said compounds (6') and (7'). See also published application EP 199,446, where compounds of the type (6') and (7') below in which R.sup.5 and R.sup.6 are taken together are similarly prepared. ##STR1##